Photolithography lens

ABSTRACT

A lens design having two concave mirrors or Mangins and a turning mirror is disclosed. The optical axis of the concave elements are approximately coplanar and intersect on a turning mirror, and the normal to the surface of the turning mirror bisects the angle between the two optical axis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application Ser.No. 60/184,868, filed Feb. 25, 2000 and is a continuation of U.S.Non-Provisional application Ser. No. 09/534,952, filed Mar. 24 2000, nowU.S. Pat. No 6,342,967, issued Jan. 29, 2002.

FIELD OF THE INVENTION

The field of the invention is the field of optical lenses, and inparticular optical lenses for use in photolithography in thesemiconductor industry.

BACKGROUND OF THE INVENTION

Advances in computers have largely been driven by advances inphotolithography, where better lenses and shorter wavelength light allowexposure of ever finer features. In large part, the lens design processis also driven by the advances in computers, as it becomes ever fasterto trace rays and numerically optimize lens design. A great deal ofscope for the experienced and talented lens designer still remains,however, as the computer does not yet “know” which of the interrelatedvariables to optimize first, nor which variable is less or moresensitive to variations in manufacture or adjustment.

In particular, as the photolithographic wavelength shrinks from around250 nanometers (nm) to 190 nm and to 157 nm to take advantage ofavailable laser sources of light, the number of adjustable parametersavailable to lens designers shrinks even faster. In the visible and nearultraviolet region, there are a multitude of glasses which have therequired homogeneity and transparency and which can provide a range ofindex of refraction and dispersion coefficients needed for productionhigh numerical aperture (N. A.) diffraction limited lenses. Fusedsilica, the material of choice for lenses, starts to absorb at 160.5 nm,and at 157 nm the absorption in the lens material would preclude use.The only practical material for the construction of refractive lensesfor the shorter wavelengths is a fluorite material, and in particularcalcium fluoride.

Refractive lenses limited to only one material need a very large numberof elements, (above 35 in some cases for flat field lenses having alarge image size and high N. A.), and need a very large diameter. Thecost of such lenses increases at least as the 3.8 power of the diameter,and the material cost and the fabrication costs become prohibitive forlenses with many elements and large dimensions.

Such lenses are now widely used in photographic and projectingequipment, television cameras, microscopy, and as of late, manufacturingequipment in the semiconductor industry. Here, anastigmatic lenses areused in step-and-repeat or scanning cameras for patterningmicroprocessors, memory and logic chips, etc. These optical systems canbe divided into two classes. The most common class is the extension ofthe double Gauss lens due to Glatzel (Zeiss Company) reported at theInternational Lens Design Conference, Mills College, Calif. in 1981, andpublished by SPIE and OSA. The Glatzel lens exhibits a “double bulge”.In the Glatzel designs, the field curvature is corrected by two or moreshrinks and expansions of the bundle of rays passing through the lens.This compares with just a single shrink in the conventional photographiclens, such as a triplet or a double Gauss lens. Such lenses areextremely expensive because many more lens elements are used. The otherclass of lenses used are the “ring systems”, where the optics iscorrected along an annulus and no effort is made to correct the fieldcurvature. Ring system lenses are inefficient in their use of the light.Ring systems have an advantage over symmetric lens systems that lightdoes not propagate through the entire volume “seen” by the lenselements, and optical elements such as'turning mirrors may be placed onor near the optical axis to act as turning mirrors. Reflective lenselements are inherently achromatic, and may be used at the shortestwavelengths. However, the aberrations introduced by reflection fromcurved surfaces at non normal incidence have convinced many designersskilled in the art that such mirrors can not give diffraction limitedperformance at high numerical apertures in an axially symmetric system.

One telescope designer, D. D. Maksutov, published a design of anaplanatic telescope in a book called ASTRONOMICZESKAYA OPTIKA, publishedby OGIZ (Leningrad) in 1945, using two curved mirrors. This design is aversion of the Gregory telescope where the secondary mirror is alsoconcave, and is positioned beyond the focal point of the primary mirror.By definition, an aplanatic telescope must have a correction forspherical aberration and coma. The two simultaneous quadratic equationswhich must be solved have real roots only for a rather large centralobstruction and a real image plane in front of the primary mirror. Thetelescope as designed left all aberrations and their higher orderresiduals uncorrected except for Seidel type spherical aberrations andcoma, and there is no way of correcting them further.

Useful references for Lens Design are: A. E. Conrady, Applied Optics andOptical Designs, 2^(nd) Edition, Dover Publications, in 2 volumes 1957and 1960; H. H. Hopkins, Wave Theory of Aberrations, Oxford UniversityClarendon Press, 1950; and R. R. Shannon, The Art and Science of LensDesign, Cambridge University Press, 1997.

The above identified references, patent applications, and provisionalpatent applications are hereby incorporated in their entirety herein byreference.

The present invention shows the way to use combinations of refractiveand reflective lens elements to produce the required lens designs.

OBJECTS OF THE INVENTION

It is an object of the invention to produce a diffraction limitedoptical lens having a high numerical aperture for wavelengths where thenumber of optical materials for refractive lens elements is small. It isa further object of the invention to produce a diffraction limitedoptical lens having a high numerical aperture having fewer lens elementsat less cost than a refractive lens having equivalent parameters. It isa further object of the invention to produce a diffraction limitedoptical lens having a high numerical aperture which includes refractiveelements and reflective elements with curved reflective surfaces. It isa further object of the invention to produce a diffraction limitedoptical lens having a high numerical aperture having two reflectiveelements, each of which receives light from an object planesymmetrically with respect to an optical axis normal to the reflectivesurface.

SUMMARY OF THE INVENTION

The present invention is a system, apparatus and method to use twofocusing mirrors in a high numerical aperture, non-ring, lens design.The optical axis of each mirror are coplanar and intersect at theposition of a turning mirror having a normal to the surface of theturning mirror which bisects the angle formed by the two optical axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Optical layout of the most preferred embodiment of theinvention.

FIG. 2. Optical layout of a preferred embodiment of the invention.

FIG. 3. Optical layout of a preferred embodiment of the invention.

FIG. 4. Schematic diagram of a system using an embodiment of theinvention.

FIG. 5. Example of a partially optimized lens design of the apparatus ofthe invention.

FIG. 6. Wavefront aberrations at 193 nm for the design of FIG. 5.

FIG. 7. Intersection of beam cones on the elliptical face of the turningmirror of FIG. 5.

FIG. 8. Example of a partially optimized lens design of the apparatus ofthe invention.

FIG. 9. Wavefront aberrations at 193 nm for the design of FIG. 8

DETAILED DESCRIPTION OF THE INVENTION

The present invention consists of a novel extended flat fieldanastigmat. In addition to having an excellent correction it has a fewernumber of elements as compared to the conventional refracting lens. Thereduction in the number of elements is fundamentally due to the way inwhich the field curvature is corrected.

As is well known in the field of geometrical optics, a reflectingsurface has a refractive index of −1. All optical materials in thevisible and ultraviolet range have a positive refractive index. Sincethe field curvature depends only on the power of the lenses, which aretaken to be infinitely thin for this purpose and refractive index, itappears that a suitable combination-of lenses and mirrors should rendera solution suitable for a vast number of applications.

FIG. 1 shows an optical diagram of the non-ring lens of the presentinvention. Light rays 11 are shown arising from the center of an object10 proceeding to a focusing mirror 12. Light rays arising from theobject 10 form a diverging beam towards the mirror 12. The light rays 11are shown reflected from mirror 12 and being focused on to a turningmirror 13. The turning mirror 13 forms a central obstruction to thelight rays 11. The rays reflected from the turning mirror 13 diverge toa second mirror 14, and are reflected as a beam to a lens 15, where theyare focused on to an image receiver 16. The turning mirror 13 forms acentral obstruction to the beam from mirror 14 to lens 15. The opticalaxis (here defined as a line through the center of an optical elementwhich is normal to the surface of the optical element at theintersection point ) of the focusing mirrors 12 and 14 are coplanar inthe most preferred embodiment of the invention, but are offset fromco-planarity in some embodiments. In the most preferred embodiment, theoptical axis of mirror 12 and 14 are coplanar, the normal to the surfaceof the turning mirror 13 bisects the angle between the optical axis ofmirror 12 and the optical axis of mirror 14. In the most preferredembodiment shown in FIG. 1, the optical axis of mirror 12 intersects thesurface of the object 10 normally in the center of the object surface.The optical axis of the mirror 14 in the most preferred embodimentintersects the center of the image receiver 16 normally. In thepreferred embodiments that the optical axis of the focusing mirrors donot intersect in the center of the object or image receiver, the opticalaxis are not in general co-planar, and there is no intersection point ofthe two optical axis. However, one of skill in the art will understandthat a turning mirror may still be used to accept the converging lightfrom focusing mirror 12 and send a diverging beam to focusing mirror 14,and that the two focusing mirrors are used to form a “double bulge” inthe bundle of light rays sent from the object plane to the image plane.Astigmatism which is introduced into the system in such a scheme ispartially compensated if the plane formed by the optical axis of mirror12 and the normal to the center of the object 10 is approximately atright angles to the plane formed by the optical axis of mirror 14 andthe normal to the center of the image receiver 16.

FIG. 2 shows that, with the addition of a single turning mirror 20 , thedesign of FIG. 1 may be modified so that the surface of the object andthe surface of the image of the object are parallel, which would berequired by practical considerations in a photolithographic steppersystem.

FIG. 3 shows that, with the addition of two turning mirrors 20 and 30,the design of FIG. 1 may be modified so that the normal to the objectsurface in the center of the object and the normal to the image surfacein the center of the image An existing “straight through” refractivelens could be removed and replaced with the lens of the invention, or,more importantly, an existing lithographic stepper design would not haveto be substantially modified to use the lens of the invention.

FIG. 4 shows an example of a lithographic system for exposingphotoresist covered wafer 40 with light transmitted through a mask 41.The wafer is held by an adjustable wafer holder 42, and the mask is heldby an adjustable mask holder 43. The wafer and mask may be adjusted withrespect to each other and with respect to the lens of the invention bymeans well known in the art. A light source 44 is shown illuminating themask 41. Light may be transmitted through the mask, as shown, or may bereflected from the mask. A computer is 45 is shown for controlling thelight source and the adjustments of the mask and wafer holders.

FIG. 5 shows an example of a partially optimized lens design of theapparatus of the invention. In FIG. 5, one of the focusing reflectingelements 50 is a Mangin, which is a transparent lens element which iscoated on one side with a reflecting material. A Mangin gives muchbetter protection to the reflective coating than the normal firstsurface reflective coating of an optical element. Each refractiveoptical element shown in FIG. 5 would normally have surfaces coveredwith a multilayer coating so that the reflectivity from the surface ofthe transparent refractive material is greatly reduced. The position 52of an aperture stop is noted.

The particular design depicted in FIG. 5 has a Numerical Aperture (NA)of 0.690 and an input NA of 0.138 giving a reduction of 5×. The lens iscalculated to form an image in a 7 mm by 7 mm area. By scaling up ordown the lens, the image field may be anywhere between 1×1 mm and 50×50mm. For lower NA lenses, the image field size may be even greater. Therefractive index of silica at 193 nm is 1.55077, and that index has beenused in the calculations. Present designs are calculated at thenormalizing wavelength of 0.193 nm. Should it be desirable to use thissystem at 157 nm or at any other wavelength down to 120 nm and up to thefar infrared spectral region, only small changes in design will berequired. For instance, CaF₂ has an index of refraction of 1.5570 at 157nm, which is very close the index of fused silica at 193 nm. Adjustmentsmay be accomplished by some very slight changes to the curvatures of theelements. Similarly in the visible range, where the wavelength is muchlonger, the aberrations will decline further. In the visible region,most of the conventional glasses may be used. The change in wavelengthdoes not change the transverse aberrations but the wavefront aberrationsdo increase in proportion to the inverse of the wavelength. Thus fordiffraction limited performance, it is essential to use suitablecriteria based on physical optics.

The attainable performance for the lens of FIG. 5 may be judged fromFIG. 6, which shows the wavefront aberrations (optical path differences)at 193 nm for the design of FIG. 5. Full scale runs from −0.1 to +0.1waves. It may be seen that only a negligible portion of the wavefrontdeparts by more then 10 nm from a perfect sphere converging on the imageplane. The Root Mean Square (RMS) value of the wavefront deformationover the field varies between 0.028 to 0.010 waves. The calculatedModulation Transfer Function (MTF) for the system and the diffractionlimited MTF show that the state of correction is almost perfect. Thelens cuts off at 7,170 lines/mm over the entire field. The lens alteredfor use at 157 nm and CaF2 would be capable of printing lines oflinewidth LW

LW=k ₁*Wavelength/NA=0.4*157 nm/0.69=90 nm

where k₁ is a proportionality constant of order 1, depending on themodulation required by a specific process.

The distortion of the lens of FIG. 5 is less than 0.002%. The lens isalso almost telecentric on the object and image size. This means thatthe off axial bundles are leaving the object plane and impinging on theimage plane almost at right angle.

As may be seen from the drawing of FIG. 5 and tabulated data of Table Iand Table IA, some adjacent radii are very close in value. For lessdemanding applications the air spaces 31 and 19 may be eliminated orsubstituted by other degrees of freedom. Also some thicknesses may bethinned or thickened depending on the results required.

The system as described has a small central obstruction of less than ⅕in linear terms. Due to this obstruction the MTF drops somewhat in thelow frequency range and is higher in the higher frequency range. Thisphenomenon is well known to the astronomers. However for printing offine lines, a small central obstruction may be somewhat beneficial,unlike in astronomy.

To keep the central obstruction within reasonable bounds a compromisemust be established between the primary reduction, which is the ratio ofthe image size in the proximity of the turning mirror to the objectsize, and the inclination of the turning element, which bisects bothoptical axes. If this angle is too large the size of the turning elementgrows rapidly due to the divergence of the beams reflected from thiselement. In this design less then 4.0% of all rays are obstructed.

Another novel feature of the design, is the position of the aperturestop. In conventional optical systems there is a field lens in theintermediate image plane; without the field lens the position of thecentral obstruction wanders over the field. Also in conventional mirrorsystems the aperture stop is positioned on a reflecting surface in orderto minimize its size. Here, to the contrary, the aperture stop is infront of the first reflecting element. Now the two focusing reflectingelements act as a “virtual field lens” imaging the aperture stop in theproximity of the turning element and thus minimizing the shift of thecentral obstruction over the field.

Another aspect of minimizing the obstruction is the focusing of theaxial bundle on the turning element. The best shape of the turningelement is not an ellipse. A small improvement in the lens design can begained by plotting the intersection of the beam cones on the ellipticalface of the turning mirror 70 as done in FIG. 7 for rays from thecenter, from each corner, and from middle of each side of a 35 by 35 mmobject (for example a lithographic mask). The central obstruction maythen be reduced by trimming away that material not needed to reflect allthe light. In some applications, it may be desirable to limit thecentral obstruction further or, as for instance in a scanning systemwith a rectangular field of view, trim away half of the turning mirror.

Table I and IA give the parameters of the various optical elements shownin FIG. 5 in a form recognizable to one of skill in the art of lensdesign who uses various lens design computer programs for ray tracing.The results of the lens calculations show that performance comparable torefractive lens design is possible using many fewer optical elements,and where the largest diameter optical elements are focusing mirrors.

FIG. 8 shows a second example of a partially optimized lens design usingthe principle of field curvature compensation and its application toflat field anastigmats.

Tables II and IIA give the parameters of the various optical elementsshown in FIG. 8 in a form recognizable to one of skill in the art oflens design who uses various lens design computer programs for raytracing.

The structure of FIG. 8 is substantially different from the previousexample shown in FIG. 5. First, the Mangin is replaced by a mirror andanother element is added close to the object plane. Also one element isremoved from the proximity of the image plane. In addition, thestructure of the refracting portion is quite different. It is possibleto design therefore a set of lenses of similar performance having incommon only the three reflecting elements and one or two groups ofrefracting elements. The system reduction is again 5× and the imageforming NA is 0.67. The image field coverage is increased to 10 mm×10mm. The lens of FIG. 8 has physical size is larger by some 30% than thelens of FIG. 5. The total number of refracting elements is the same. Oneelement is added to the object lens group and one element is removedfrom the image forming group. Also one aspheric is added on a negativesurface. The lens data are again calculated for 193 nm and therefractive index of silica.

FIG. 9 shows the wavefront aberrations (optical path differences) forthe design of FIG. 8 with full scale −0.1 to +0.1 waves at 193 nm. Itmay be seen that only a negligible portion of the wavefront departs bymore then 10 nm from a perfect sphere converging on the image plane.

The RMS wavefront error as a function of the field. In the mid-field itis below {fraction (1/100)} of a wavelength. The wave aberrations arebelow {fraction (1/20)} of a wave and therefore the lens may be alsoused at 157 nm. The MTF is also excellent and the distortion is lessthan ⅓ of the first design. The other properties are similar to thedesign in FIG. 5

The design of FIG. 8 may be used to explain the performance of thesystem and why so few optical elements are required to obtain it. Thefield curvature contributed by the two aspheric mirrors is −242.66waves. The refracting elements have a contribution of +241.58 wavesgiving a total of −1.08 waves of Petzval sum or field curvature forcompensation of higher order curvatures. In other words, the contractionand expansion of the rays needed to accomplish the correction is now dueto the rays coming to focus between the two reflecting elements. Usingrefractive elements for this purpose would make the field curvatureworse.

This new family of designs has a fundamental advantage in limiting thesize of the refracting elements. It should be noted that since thenumber of these surfaces is smaller the geometrical accuracy of thesesurfaces may be somewhat lower than in traditional step-and-repeat lensand hence each lens element will be cheaper to produce than the lenselements of the traditional lenses. The aspherics, if any, can be placedon concave surfaces and thus are easier to measure correctly.Furthermore, the rotational symmetry of the system makes assembly andtesting easier unlike in the case of ring systems. The only negativeattribute of the present invention is that the smaller elements oflarger bending power have tighter centering and spacing requirementsthan the requirements of conventional lenses. These tighter requirementsare the direct outcome of the small number of refracting elements usedin the present invention.

While both of the designs described above pertain to monochromaticapplications, the basic structure is equally suitable for use inmicroscopy, photography, and in other uses where color correction isessential. In these situations, the refractive portions of the systemmay be achromatized separately or in combination with Mangin mirrors.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

TABLE I Wavelength 193 nm, Num. Ap. 0.67, 10 mm sq. Field Surf. Radiusof Surface No. Curvature Separation Diameter Material Obj. Inf. 30.875070.70  1 368.2845 18.5250 79.80 SiO2  2 −1242.2490 14.8200 81.31  3105.3077 18.5250 84.10 SiO2  4 95.2710 24.0000 80.00  5 −193.576418.5250 81.40 SiO2  6 −297.8511 819.5419 86.19 Stop Inf. 147.5800 265.60 8 −497.8865 −322.6028 310.11 Mirror 1  9 Central Obstruction 46.94 10Coordinate Brake  (Tilt about X-axis 38 degrees) 11 Inf. EllipicalMirror 50.58  (Approx. Size: X half width 13.5 mm,  Y half width 25.3mm) 12 Coordinate Brake  (Tilt about X-axis 38 degrees) 13 Centr. Obstr.323.1437 46.94 Mirror 2 14 −478.3385 −323.1437 333.12 15 Inf. −137.0957206.65 (Dummy) 16 −286.1025 −17.2900 165.02 SiO2 17 −5762.3040 −1.1115162.96 18 −148.0280 −13.2727 151.15 SiO2 19 −193.7651 −1.2310 145.94 20−138.6282 −32.6773 121.46 SiO2 21 −211.4544 −19.7600 113.94 22 428.6247−23.9077 116.28 SiO2 23 603.1213 −7.4100 100.72 24 146.6552 −11.1150100.01 SiO2 25 156.9977 −2.7341 96.70 26 −287.1838 −22.3328 84.61 SiO227 471.6131 −0.6175 72.71 28 −101.1013 −21.7000 63.54 SiO2 29 −141.7320−0.6175 44.43 30 −132.0143 −21.7000 43.34 SiO2 31 Inf. −4.0000 22.46 Image Plane 14.14

TABLE IA Coefficients of Asphericity Coeff. Mirror 1 Mirror 2 Coeff. R4−2.6020123 E−12  2.6702455 E−12 Coeff. R6 −5.0990706 E−16 −2.3265643E−16 Coeff. R8  1.6336221 E−20 −1.4316566 E−20 Coeff. R10 −3.0639961E−25  3.8030308 E−25 Coeff. R12  7.5079931 E−30 −5.0351442 E−30 ConicConst. −0.3970695 −0.3447434 Coefficient of Aspericity Surface No 23Coeff. R4 −1.4138791 E−10 Coeff. R6  3.3135025 E−13 Coeff. R8 −1.9295018E−16 Coeff. R10  5.8356675 E−20 Coeff. R12 −6.0006468 E−24 NominalRefracting Index 1.550000

TABLE II Wavelength 193 nm, Num. Ap. 0.69, 7 mm sq. Field Surf. Radiusof Surface No. Curvature Separation Diameter Material Obj. Inf. 21.350049.00  1 270.6063 9.8000 57.00 SiO2  2 −144.6241 1.4000 57.00  3−146.1938 8.7500 57.00 SiO2  4 −1404.9790 583.8597 59.00 Stop Inf.110.3900 163.00  6 −343.951 −226.7657 196.41 Mirror 1  7 CentralObstruction 38.00  8 Coordinate Brake  (Tilt about X−axis 36 degrees)  9Inf. Ellipical Mirro 50.58 Mirror  (Approx. Size: X half width 11.5 mm, Y half width 17.7 mm) 10 Coordinate Brake  (Tilt about X-axis 36degrees) 11 Centr. Obstr. 202.7649 38.00 12 −327.3131 7.0000 193.66 SiO213 −333.0981 −7.0000 198.98 Mirror 2 14 −327.3131 −199.5970 196.00 15Inf. 0.0000 136.22 Dummy 16 Inf −82.9256 136.22 17 −103.2848 −12.6000117.81 SiO2 18 −188.1026 −0.1400 116.02 19 −185.6243 −10.5700 115.06SiO2 20 −310.4964 −0.1836 112.44 21 −100.2444 −13.2300 105.94 SiO2 22−147.3123 −16.0300 100.16 23 −347.2480 −25.9036 90.81 SiO2 24 −125.7793−1.1321 72.14 25 −64.4709 −17.2788 68.22 SiO2 26 −88.6415 −8.6100 57.0127 145.0366 −5.2633 55.81 SiO2 28 125.3962 −0.4645 53.65 29 −39.9236−17.7919 43.36 SiO2 30 −18.4713 −0.3290 24.92 31 −19.0885 −9.3300 24.84SiO2 32 1009.4860 −0.3080 19.62 33 169.6456 −3.3600 19.48 SiO2 34 Inf.−2.8000 15.70  Image Plane 9.90

TABLE IIA Coefficients of Asphericity Coeff. Mirror 1 Mirror 2 Coeff. R4 0.0000000  0.0000000 Coeff. R6 −1.5654074 E−14 −1.0295891 E−15 Coeff.R8  8.7973552 E−19  1.2620922 E−21 Coeff. R10 −1.1296997 E−23 −7.3634361E−25 Coeff. R12  5.1723374 E−25  2.0553938 E−28 Con. Const. −0.3061605−0.2981577 Nominal Refractive Index 1.550770

I claim:
 1. An apparatus for imaging light from an extended object on toan image receiver located at a finite distance from the extended object,comprising: a base; a first focusing reflective element, the firstfocusing reflective element attached to the base, the first focusingreflective element having a first optical axis, the first focusingreflective image receiving a diverging beam of light from the object; asecond focusing reflective element, the second focusing reflectiveelement attached to the base, the second focusing reflective elementhaving a second optical axis; a first turning mirror attached to thebase, the first turning mirror having a small area compared to areas ofthe first and second focusing reflective elements, the first turningmirror forming a central obstruction in the diverging beam of light; atleast one refractive element attached to the base, the refractiveelement contributing to the optical correction required to correct theimaging of the extended object on to the image receiver; wherein lightemitted from the extended object is transmitted as the diverging beam oflight to the first focusing reflective element; and wherein thediverging light beam is reflected from the first reflective element as aconverging beam to the first turning mirror, and wherein the light isfurther reflected from the first turning mirror as a diverging beam tothe second reflective element, and wherein the light reflected from thefirst turning mirror is reflected in turn from the second reflectiveelement and to the image receiver, and wherein the first turning mirrorforms a central obstruction to the light reflected from the secondreflective element to the image receiver; and wherein the light passesthrough the at least one refractive element located in at least oneposition chosen from the group of positions consisting of a positionbetween the extended object and the first focusing reflective elementand a position between the second reflective element and the imagereceiver, and wherein an image of the extended object is formed on thesurface of the image receiver.
 2. The apparatus of claim 1, where theapparatus forms a non-ring lens.
 3. The apparatus of claim 1, where thefirst turning mirror obstructs less than 10% of the diverging beam oflight.
 4. The apparatus of claim 3, where the first turning mirrorobstructs less than 6% of the diverging beam of light.
 5. The apparatusof claim 4, where the first turning mirror obstructs less than 4% of thediverging beam of light.
 6. An apparatus for imaging light from anextended object on to an image receiver located at a finite distancefrom the extended object, comprising: a base; a first focusingreflective element, the first focusing reflective element attached tothe base, the first focusing reflective element having a first opticalaxis, the first focusing reflective element receiving a diverging beamof light from the object; a second focusing reflective element, thesecond focusing reflective element attached to the base, the secondfocusing reflective element having a second optical axis non-colinearwith the first optical axis, a first turning mirror attached to thebase, where the first optical axis and the second optical axis eachintersect the reflecting surface of the first turning mirror, and wherethe first turning mirror has a small area compared to areas of the firstand second focusing reflective elements; at least one refractive elementattached to the base, the refractive element contributing to the opticalcorrection required to correct the imaging of the extended object on tothe image receiver; wherein: light emitted from the extended object istransmitted as the diverging beam of light to the first focusingreflective element in the general direction of the first optical axis;and wherein the diverging light beam is reflected from the firstreflective element as a converging beam towards the first turningmirror, and wherein the light is further reflected from the firstturning mirror and travels as a diverging beam to the second reflectiveelement, and wherein the light reflected from the first turning mirroris reflected in turn from the second reflective element and transmittedas a converging beam in the general direction of the second optical axisto the image receiver, and wherein the light passes through the at leastone refractive element located in at least one position chosen from thegroup of positions consisting of a position between the extended objectand the first focusing reflective element and a position between thesecond focusing reflective element and the image receiver, and whereinan image of the extended object is formed on the surface of the imagereceiver.
 7. The apparatus of claim 6, where the apparatus forms anon-ring lens.
 8. The apparatus of claim 6, where the first turningmirror obstructs less than 10% of the diverging beam of light from theobject.
 9. The apparatus of claim 8, where the first turning mirrorobstructs less than 6% of the diverging beam of light from the object.10. The apparatus of claim 9, where the first turning mirror obstructsless than 4% of the diverging beam of light from the object.
 11. Asystem, comprising: a base; a light source attached to the base, thelight source for illuminating an extended mask removably attached to thebase; a first focusing reflective element, the first focusing reflectiveelement attached to the base, the first focusing reflective elementhaving a first optical axis, the first focusing reflective imagereceiving a diverging beam of light from the extended mask; a secondfocusing reflective element, the second focusing reflective elementattached to the base, the second focusing reflective element having asecond optical axis; a first turning mirror attached to the base, thefirst turning mirror having a small area compared to areas of the firstand second focusing reflective elements, the first turning mirrorforming a central obstruction in the diverging beam of light; at leastone refractive element attached to the base, the refractive elementcontributing to the optical correction required to correct the imagingof the extended mask on to a photoresist covered semiconductor wafer;and a mask aligner system for aligning the extended mask with respect tothe photoresist covered wafer; wherein light emitted from the extendedmask is transmitted as the diverging beam of light to the first focusingreflective element; and wherein the diverging light beam is reflectedfrom the first reflective element as a converging beam to the firstturning mirror, and wherein the light is further reflected from thefirst turning mirror as a diverging beam to the second reflectiveelement, and wherein the light reflected from the first turning mirroris reflected in turn from the second reflective element and to thephotoresist covered wafer, and wherein the first turning mirror forms acentral obstruction to the light reflected from the second reflectiveelement to the photoresist covered wafer; and wherein the light passesthrough the at least one refractive element located in at least oneposition chosen from the group of positions consisting of a positionbetween the extended mask and the first focusing reflective element anda position between the second reflective element and the photoresistcovered wafer, and wherein an image of the extended mask is formed onthe surface of the photoresist covered wafer.
 12. The system of claim11, where optical elements form a non-ring lens system.
 13. Theapparatus of claim 11, where the first turning mirror obstructs lessthan 10% of the diverging beam of light.
 14. The apparatus of claim 13,where the first turning mirror obstructs less than 6% of the divergingbeam of light.
 15. The apparatus of claim 14, where the first turningmirror obstructs less than 4% of the diverging beam of light.